The present invention claims priority to U.S. Provisional Application No. 61/020,132 filed Jan. 9, 2008. The present invention relates generally to integrated circuit (IC) designs, and more particularly to a voltage-control oscillator circuits with combined MOS/bipolar device for various circuit design applications.
A voltage-controlled oscillator (VCO) is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The VCO generates a signal varying at two predetermined voltages levels at a frequency controlled by the voltage input. The VCO can be found in various circuits, such as phase-locked loops in communication applications. These phase-locked loops are able to generate stable frequencies, recover a signal from a noisy communication channel, or distribute clock timing pulses in digital logic designs.
FIG. 1A schematically illustrates a conventional Hartley oscillator 100 comprised of a bipolar transistor 102, a resistor 110, a capacitor 108 and inductors 104 and 106 for the purposes of explaining the operation of a simple electronic oscillator. The bipolar transistor 102 has a collector connected to an end of the resistor, an emitter connected between the inductors 104 and 106, and a base connected to an end of the capacitor. The resistor 110 is connected between the collector of the bipolar transistor 102 and the inductor 106 or the capacitor 108. The inductors 104 and 106 are serially connected, with one end of the inductor chain connected to the base of the bipolar transistor 102 and the capacitor 108, and another end connected to the resistor 110 and the capacitor 108. The capacitor 108, the chain of inductors 104 and 106, and the resistor 110 are arranged in parallel.
In operation, a bias is applied to the base of the bipolar transistor 102 for controlling the amplified current output from the emitter. The amplified current from the emitter is directed back to the base to induce the next cycle of current amplification. The inductors 104 and 106, the capacitor 108, and the resistor 110 make up a LCR circuit that provides the bipolar transistor 102 with impedance to adjust the frequency of the current amplification cycles of the bipolar transistor 102. Changing the impedance results in a change of the oscillation frequency.
FIG. 1B is a schematic diagram illustrating a voltage-controlled oscillator (VCO) commonly used in radio frequency (RF) devices for telecommunication applications. Serially connected inductor 152 and capacitor 164 form a LC oscillator with a NMOS transistor 172 periodically “on” and “off” to control the discharging and charging, respectively, of the capacitor 164. Symmetrically, serially connected inductor 154 and capacitor 162 form another LC oscillator with a NMOS transistor 174 periodically “on” and “off” to control the discharging and charging, respectively, of the capacitor 162. A gate of the NMOS transistor 172 is connected to a node B. Voltage at the node B increases as the capacitor 162 is charging up. When the voltage at the node B passes a threshold voltage of the NMOS transistor 172, the NMOS transistor 172 will be turned “on” and cause the capacitor 164 to discharge. Symmetrically, a gate of the NMOS transistor 174 is connected to a node A. Voltage at the node A increases as the capacitor 164 is charging up. When the voltage at the node A passes a threshold voltage of the NMOS transistor 174, the NMOS transistor 174 will be turned “on” and cause the capacitor 162 to discharge. In order to control the frequencies of oscillators, a controllable voltage is applied to the capacitors 162 and 164 at a node VTUNE. Capacitances of the capacitors 162 and 164 vary, so are the frequencies, proportionally to the changes of the voltage at the node VTUNE. In fact, the capacitors 162 and 164 with voltage controlled variable capacitance are called varactors.
However, varactors are conventionally formed by a vertical structure in a bipolar process while virtually all other devices in a chip are manufactured in a CMOS processes. Separate bipolar process steps add cost and complexity to the CMOS process. As such, what is desired in VCO with simple device structures and a manufacturing process compatible with the common CMOS process.